The present invention relates to a dehydration-sintering furnace, and specifically relates to a dehydration-sintering furnace which performs dehydration processing and sintering processing of an optical fiber (glass fiber) base material in fabrication of an optical fiber.
A porous base material is produced by use of a VAD (Vapor phase Axial Deposition) method, an OVD (Outside Vapor Deposition) method, or the like. A dehydration-sintering furnace includes a core tube (muffle) which houses the porous base material, and a heater. The porous base material is heated and dehydrated, with a dehydrating agent such as chlorine gas and thionyl chloride, and an inert gas such as helium gas supplied into the core tube. Thereafter, the porous base material is heated and sintered in the inert gas atmosphere such as helium gas (another gas may be included, if needed), thus obtaining the optical fiber base material.
The dehydration-sintering furnace, which performs dehydration and sintering of the optical fiber base material as described above, includes the core tube. The core tube is connected to a pressure-variation damper, in order to avoid a large variation in an internal pressure of the core tube, and to keep the internal pressure of the core tube as constant as possible. The pressure-variation damper includes, for example, a balloon-type pressure buffer, a gas flow buffering chamber, a solenoid valve-controlled pressure-variation absorbing container, and the like (for example, Patent Documents 1 to 4: Patent Document 1: Utility Model Application Laid-Open Publication After Examination No. Hei 6 (1994)-50513; Patent Document 2: Japanese Patent Laid-Open Publication No. Hei 5 (1993)-4828; Patent Document 3: Japanese Patent Laid-Open Publication No. Hei 6 (1994)-127964; Patent Document 4: Japanese Patent Laid-Open Publication No. Hei 10 (1998)-120428).
Although the pressure-variation damper connected to the core tube is effective in absorbing a short-period pressure variation in the core tube, the pressure-variation damper has the following problems.
When the pressure in the core tube is high, gas in the furnace is pushed out of the core tube into the pressure-variation damper. On the other hand, when the pressure in the furnace drops, gas in the pressure-variation damper is returned into the core tube.
Specifically, in a dehydration processing step, removed moisture moves into the pressure-variation damper, and a part of the moisture is returned into the core tube. This deteriorates a dehydration ability.
Such a phenomenon has not heretofore become a significant problem. However, the phenomenon raises a problem, for example, in the following two cases. The first is the case of fabricating a recently increasingly demanded optical fiber with a reduced OH peak which appears at a wavelength of about 1385 nm in a transmission loss curve of the optical fiber (for example, Patent Document 5: Japanese Patent Laid-Open Publication No. 2003-167144). The second is the case of performing dehydration and sintering of a large-size base material having a large absolute amount of moisture contained in a porous base material.